Impact of Hydrogen Exposure and Pressure Cycles on the Geomechanical and Flow Properties of Corsehill and Bentheimer Sandstones

Underground hydrogen storage (UHS) in porous reservoir presents a promising solution for renewable energy storage; however, its safety and sustainability partly depend on its effects on the geomechanical stability and flow properties of reservoir rocks under injection-withdrawal cyclic operations. This study evaluates the effects of hydrogen exposure and pore pressure cycles including the rate and the number of cycles on reservoir sandstones. These experiments were carried out on two sandstones representing two different lithologies: Corsehill sandtone (clay rich), and Bentheimer sandtone (99% quartz).  Rock plugs were exposed to hydrogen at 70°C and 18 MPa for 50 days, with nitrogen-exposed and unexposed samples used as controls to isolate hydrogen-specific effects. To evaluate impacts on geomechanical and flow properties, triaxial and flow tests were performed before and after each pore pressure cycle at in-situ reservoir stresses and temperatures relevant for reservoirs in the North Sea.  

The geomechanical results revealed a progressive decrease in stiffness in Corsehill sandstones with increasing pressure cycles.  This trend is more pronounced in hydrogen-exposed samples and samples undergoing slow pressure cycling. Specifically, the stiffness of Corsehill sandstone decreased by approximately 10% after 15 pore pressure cycles. In contrast, Bentheimer sandstone exhibited no substantial mechanical changes under cyclic loading, with the change being approximately 1%. Although gas-exposed samples showed higher stiffness compared to unexposed ones, highlighting a mechanical effect of gas exposure, no noticeable effect could be attributed to hydrogen exposure alone.

Flow tests results indicated a progressive decline in permeability for Corsehill sandstone with increasing cycles, with faster cycles causing a more pronounced reduction compared to slower cycles. In contrast, Bentheimer sandstone showed varying trends: unexposed samples experienced an increase in permeability with increasing cycles, while gas-exposed samples exhibited a reduction. Notably, Bentheimer sandstone displayed a greater reduction in permeability during slow cycles compared to fast cycles. 

These findings show the critical role of lithology, hydrogen exposure, and cyclic loading in determining the geomechanical and flow behavior of reservoir rocks. The pronounced decrease in permeability and stiffness in Corsehill sandstone compared to the stability of Bentheimer sandstone highlights the need for detailed evaluations of the targeted reservoirs and injection strategy in UHS operations. 

Keywords: Underground hydrogen storage, geomechanical parameters, permeability, cyclic loading, hydrogen exposure